docs: def save(self):
          """Save the manual clustering back to disk."""
          spike_clusters = self.clustering.spike_clusters
          groups = {c: self.cluster_meta.get('group', c) or 'unsorted'
                    for c in self.clustering.cluster_ids}
          # List of tuples (field_name, dictionary).
          labels = [(field, self.get_labels(field))
                    for field in self.cluster_meta.fields
                    if field not in ('next_cluster')]
          # TODO: add option in add_field to declare a field unsavable.
          self.emit('request_save', spike_clusters, groups, *labels)
          # Cache the spikes_per_cluster array.
          self._save_spikes_per_cluster()